The invention relates to improving the capacity of data transfer in a communication system, or more specifically a mobile communication system.
FIG. 1 shows, from the point of view of the invention, the essential parts of a cellular mobile communication system. Mobile stations (MS) communicate with base transceiver stations (BTS) over an air interface Um. The base stations are controlled by base station controllers (BSC) which are connected to mobile switching centers (MSC). A subsystem under control of a base station controller BSC, including the base stations BTSn it controls, is commonly referred to as a base station subsystem (BSS). The interface between the mobile switching center MSC and the base station subsystem BSS is referred to as an A-interface. The part of the mobile communication system at the MSC side of the A-interface is referred to as a network subsystem (NSS). Correspondingly, the interface between the BSC and the BTS is referred to as an Abis interface. The mobile switching center MSC handles the connecting of incoming and outgoing calls. It performs functions similar to those of an exchange of a public switched telephone network (PSTN). In addition to these, it also performs functions characteristic of mobile communications only, such as subscriber location management, jointly with the subscriber registers VLR and HLR of the network. As an alternative to the circuit switched connection described above, the connection to the mobile station MS may also take place via a packet network GPRS (General Packet Radio Service).
One of the growing fields of application for mobile stations is to establish data links in connection with portable computers. Such a computer is represented by the computer PC in FIG. 1. The computer PC and the mobile station MS may be separate units or they may form an integrated whole. The data to be transmitted on data links is assembled into frames (F) that typically contain a header section 1 and a data section 2. If an increase occurs in the number and/or use of mobile stations, a bottleneck will be met in the form of the transfer capacity of the air interface Um. The transfer capacity of the air interface may be increased by compressing the data to be transmitted over the air interface. The compression is based on some bit patterns in the data stream being frequent and some occurring only once. The bit patterns which occur frequently may be sent only once as a whole, and later it is possible to send only a reference to the bit pattern sent earlier. A number of such compression algorithms has been developed,-including RLE (Run Length Encoding) and LZ (Lempel-Ziv) with its different variants, JPEG, MPEG etc. Within the scope of this application, compression ratio of an algorithm refers to the ratio between the length of an uncompressed bit string (e.g. a frame) and the length of the bit string compressed with the algorithm in question.
The conventional packet communication described above encounters the problem that data of a specific type may be compressed more efficiently with a particular compression algorithm whereas data of some other type may be compressed more efficiently with another algorithm. Also such bit patterns exist that cannot be compressed with any algorithm, whereby the identifier indicating the algorithm used just adds to the length of the bit string. A further problem with the prior art compression carried out with a fixed algorithm is that is does not offer optimal protection against eavesdropping because the algorithm does not change.
For example, PCT application WO 94/14273 discloses a system for compressing data to be transmitted with a number of different compression means. However, the WO 94/14273 application mainly relates to transmission of video information to several receivers simultaneously. Because the same information is transmitted to several receivers, such a system does not comprise a bottleneck comparable to the air interface of a cellular packet radio network wherein unique information is transmitted between each pair of transmitter and receiver. Also the WO 94/14273 application assumes that the best compression method can be determined by the contents of the transmitted information. This assumption may be correct if it is known before transmission that the information will be video information.
It is consequently the object of the invention to develop a method by means of which the limited capacity of the air interface or another low-speed telecommunication resource may be utilized as efficiently as possible, thereby simultaneously enhancing traffic encryption against unauthorized listening. The objects of the invention are achieved by a method which is characterized by that which is set forth in the independent claims. The preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on the notion that in a general case, when the contents of the data to be transmitted may be arbitrary, the most efficient compression algorithm can be established only experimentally. Because of this, the data to be transmitted are, according to the invention, compressed with a number of different algorithms, and the best of the compression results is transmitted to the receiving party. The invention is further based on the view that it is worth while in a telecommunication system which contains a slow telecommunication resource, such as an air interface limiting the capacity, to carry out a lot of extra computation in a location where that is cheap and where capacity enlargements may result from such an action.
The method according to the invention utilises the bandwidth of the bottleneck (such as the air interface in a telecommunication system) in the most effective manner possible because the compression algorithm is selected on the basis of actual testing between different algorithms. The method is applicable to several types of telecommunication systems. The method according to the invention may be used adaptively so that the transmitting party learns to apply a specific algorithm on a specific connection. A further advantage of the inventive method is provided by improvements regarding the protection of telecommunication as an eavesdropper will have difficulties in interpreting a message whose compression algorithm may change in the middle of the connection, even between successive packets.